The invention relates to a flash lamp with a gas fill for suppressing self-starting, comprising an elongate discharge tube having two electrodes arranged in the discharge tube at opposite ends of the discharge tube, wherein a starting electrode, to which a starting voltage can be applied, is arranged outside the discharge tube, and wherein the discharge tube has a length of at least 1000 mm and is filled with a gas fill.
In order to start an axial flash lamp with a length which is orders of magnitude greater than the diameter, consisting of a discharge tube with two electrodes (cathode and anode), generally three different methods are used, namely series, parallel or external starting. In the case of series or parallel starting, a transformer is connected in series or in parallel with the electrodes of the flash lamp and a high-voltage pulse or a starting voltage is supplied to the electrodes. The starting-voltage field leads to gas ionization in the flash lamp. Thus, owing to the field strength, an avalanche-like multiplication of the free charge carriers in the flash lamp takes place.
Since the required starting voltage increases with the lamp length in the case of series or parallel starting, a very high starting voltage or a very large and more expensive transformer is required for long flash lamps. In order to avoid such a large transformer, a long flash lamp is typically started externally, i.e. a starting electrode is fitted on the outer side of the flash lamp and a starting voltage is applied to said starting electrode. After starting, the gas discharge is maintained with an operating voltage at the electrodes.
In practice, however, self-starting of the flash lamp can occur, i.e. starting of the flash lamp is not caused by means of the starting voltage but by an undesired, parasitic electric field, for example. This self-starting is uncontrollable and should be prevented in various applications since it changes light exposure processes, for example.
For example, the spacing between the cathode and the anode is sufficiently large in the case of a flash lamp cooled by water with a length of, for example, 4000 mm with a filling pressure of, for example, 400 mbar, with the result that the required voltage for self-starting is far above the operating voltage of 10-45 kV. Even voltage differences of 70 kV between the cathode and the anode are insufficient for causing self-starting.
If, however, electrically conductive surfaces are applied in the vicinity of the flash lamp, for example a starting electrode in the form of a starting wire, which is used for external, controlled starting of the flash lamp, the self-starting voltage is reduced to values below 20 kV. The closer the starting wire is to the flash lamp, the more reliable the desired controlled starting of the flash lamp becomes, but also the self-starting voltage decreases with the spacing. This latter fact is explained by the acceleration of natural electrons in the flash lamp towards the starting wire which release secondary electrons on the glass wall of the flash lamp and cause an avalanche effect. Even in the case of a potential-free starting wire, for example connection of the starting wire via a capacitor to ground potential, an avalanche effect is provoked by the physical vicinity in combination with creeping discharges or leakage currents.